Yep, that's exactly what that is. The duration of the other camera's flash is shorter than a single video frame, creating "flashing banding" in a few consecutive frames (due to most cameras having a pre-flash for focus assistance). You'll get it with strobes at parties, too. It's caused from the pixels of the CMOS sensor being scanned from top to bottom one line at a time, rather than simultaneously like a CCD sensor. The effect it has on a CCD sensor is different brightnesses from frame to frame instead of banding, due to the timing of the flash. If a flash goes off at the beginning of a given frame, that frame would be brighter than the one following it where the flash only lasted for a fraction of the duration of the video frame. There's a trade-off though. CCD sensors will give light-streaks (think flashlights, candles, christmas lights, light bulbs, etc) which appear as either vertical or horizontal, edge-to-edge streaks that emit from light sources.

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"Your world is all these elements; of light and sound, of taste, smell, and touch. Woven together in many dimensions on the fabulous loom of your brain. Your brain; the most complicated thing in the world, which you yourself grew...without even thinking about it."

If you shoot many weddings, you better get the software that cleans that up for you!! I believe its new blu. It will save a lot of time. I shoot two cameras and i just swap cameras when that happens. as my second camera is a ccd camera.